EARLIEST KNOWN MAMMALIAN STAPES FROM AN EARLY CRETACEOUS EUTRICONODONTAN MAMMAL AND IMPLICATIONS FOR EVOLUTION OF MAMMALIAN MIDDLE EAR JIN MENG and SHILIN HOU Meng, J. and Hou, S.-L. 2016. Earliest known mammalian stapes from an Early Cretaceous eutriconodontan mammal and implications for transformation of mammalian middle ear. Palaeontologia Polonica 67, 181–196. The earliest known mammalian stapes with unequivocal morphology from an Early Cretaceous eutriconodontan, Chaoyangodens lii, is described. The stapes is roughly rectan- gular in dorsal view with parallel crura and equal width on both the distal and proximal ends. The stapes has a large stapedial foramen and a prominent process that is interpreted as for insertion of the stapedius muscle (PISM). The PISM is homologized with the proximal end of the interhyal, instead of the dorsal process of the sauropsid extrastapes; the latter is fused to the crista parotica of the otic capsule as the tympanohyal in mammals. The morphology of the stapes is probably characteristic of the transitional mammalian middle ear and supports the notion that the rod-like, perforated stapes with a broad end-on contact with the incus is a primitive stapes condition for mammals. During the evolution of mammalian middle ear, the distal end of the stapes and the PISM reduced the size, along with reduction of other ear ossicles. Because the stapedius muscle is absent in monotremes, the columelliform-imper- forate stapes of monotremes probably lacks the homologue of the PISM. Key words: Stapes, eutriconodontan, mammal, Cretaceous, middle ear, evolution. Jin Meng [[email protected]], Division of Paleontology, American Museum of Natural His- tory, New York, NY 10024, USA. Shilin Hou [[email protected]], Department of Geology, Northeastern University, Shenyang 110819; Paleontological Institute, Shenyang Normal University, 253 North Huanghe Street, Shenyang 110034, Liaoning Province, China. Received 21 May 2015, accepted 20 July 2015. http://dx.doi.org/10.4202/pp.2016.67_181 182 JIN MENG AND SHILIN HOU INTRODUCTION The definitive mammalian middle ear (DMME) (Allin and Hopson 1992) in mammals (Rowe 1988) dif- fers from other extant tetrapods in having a unique middle ear that consists of multiple ossicles. In contrast, the middle ear of non-mammalian amniotes has one ossicle, the stapes (or columella auris). The evolution of the DMME has been a subject that has attracted enormous attention since at least Reichert (1837) and Gaupp (1908, 1913) and still remains as an interesting topic in the study of mammalian evolution. Incorporation of the malleus-incus complex into the middle ear during the early evolution of mammals resulted in an increased bandwidth of hearing and was thus a great biological advantage in mammals (Fleischer 1978). Significant progress has been made recently in documenting middle ear morphologies in Mesozoic mam- mals, including the ossified Meckel’s cartilage, the malleus (= the articular and prearticular), incus (= quad- rate) and ectotympanic (= angular) (Wang et al. 2001; Meng et al. 2003, 2011; Luo et al. 2007; Ji et al. 2009; Luo 2011). Based on these recent discoveries, the transitional mammalian middle ear (TMME) has been hypothesized as an intermediate stage between the mandibular middle ear, as represented in Morganucodon (Kermack et al. 1973, 1981), and the DMME in all extant mammals (Allin and Hopson 1992). In addition to new discoveries made in paleontology, evolution of the mammalian middle ear has also continued to attract considerable attention from developmental and molecular studies that focus on the ho- mology and gene expression of middle ear ossicles and related structures (Maier 1990; Zeller 1993; Mallo 1998, 2001, 2003; Sánchez-Villagra et al. 2002; Tucker et al. 2004; Wilson and Tucker 2004; O’Gorman 2005; Takechi and Kuratani 2010; Anthwal et al. 2013; Gillis et al. 2013; Kitazawa et al. 2015). More inter- estingly, paleontological findings and results of developmental biology converge in revealing not only the homology of the middle ear ossicles but also development of structural details of the ossicles, such as the manubrium as a neomorphic outgrowth of the malleus (Allin and Hopson 1992; Clack and Allin 2004; Meng et al. 2011; Anthwal et al. 2013). However, the aforementioned developmental and paleontological studies have focused primarily on the ear ossicles that are transformed mandibular elements of non-mammalian cynodonts. In contrast, the stapes of Mesozoic mammals remains little known and less investigated, as already pointed out by Novacek and Wyss (1986). Although the stapes was discovered in the mid 16th century (Mudry 2013), its homology is still a controversial subject in developmental and evolutionary biology. The stapes has been known as the most conservative element of the middle ear ossicles, with its essential form and ontogeny being traceable across vertebrates (Goodrich 1930; Eaton 1939; Romer 1941; Westoll 1943; Parrington 1949, 1955, 1979; Tumarkin 1968; Fleischer 1978; Lombard and Bolt 1979; Novacek and Wyss 1986; Allin and Hopson 1992; Clack and Allin 2004). As Eaton (1939, p. 117) concluded: “The crossopterygian hyomandibular bone, evolving into the tetrapod stapes, retains its original morphological relations largely unchanged, even to mammals.” In addition, the articulation between the stapes and incus in mammals represents the quadrate-hyomandibular joint that was present in ancestral amniotes (Kitazawa et al. 2015). The stapes is rarely preserved in early mammals because of its small size and fragility. Among known Mesozoic mammals, there is one nearly complete stapes repor ted from a Late Cretaceous eutherian (Archibald 1979). Fragments of the stapes were reported from the Late Cretaceous multituberculate Kryptobaatar (Rougier et al. 1996a) and Chulsanbaatar (Hurum et al. 1996) and from Early Cretaceous eutriconodonts (Luo et al. 2007; Meng et al. 2011). Except for the stapes from a Late Cretaceous eutherian (Archibald 1979), the unambiguous morphology of the stapes remains poorly known in Mesozoic mammals and most of their close relatives, such as Haldanodon (Lillegraven and Krusat 1991; Ruf et al. 2013), Morganucodon (Kermack et al. 1981), and Sinoconodon (Crompton and Luo 1993). As yet, there is no convincing evidence of the stapes known from eutriconodontans. Because of the scant fossil record, the ancestral condition of the stapes in mammals and transformation of the mammalian stapes during evolution of the mammalian middle ear has remained contradictory (Novacek and Wyss 1986; Meng 1992). It is posited that the rod-like, perforated stapes is the most likely ground plan for derivation of the mammalian stapes and that it probably persisted in “triconodonts” because of its com- mon occurrence in cynodonts and other tetrapods (Novacek and Wyss 1986). Here we report the stapes with unequivocal morphology from the Early Cretaceous eutriconodontan Chaoyangodens lii Hou et Meng, 2014. It is the earliest known mammalian stapes, given current phylogenetic placements of the eutriconodontans within Mammalia (Luo et al. 2002, 2007; Kielan-Jaworowska et al. 2004; Rowe et al. 2008; Yuan et al. 2013; Zheng et al. 2013; Bi et al. 2014; Krause et al. 2014). The morphology of the stapes supports the notion EARLIEST KNOWN STAPES OF EUTRICONODONTAN MAMMAL 183 that the rod-like, perforated stapes is a primitive condition for mammals. It provides new evidence about the stapes morphologies during the evolution of the mammalian middle ear, from the mandibular middle ear to the TMME and then to DMME. It also raises the issue on the homology of the process for insertion of the stapedius muscle in therians, a feature to which little attention has been paid. Institutional abbreviations. — JTZ, Ji-Zan-Tang Paleontological Museum, Chaoyang, Liaoning Province, China; MGL, Museu Geológico de Lisboa, Lisbon, Portugal. Acknowledgements. — We thank Lianhai Hou (Institute of Vertebrate Paleontology and Paleoanthro- pology, Beijing, China) and Haijun Li (Curator of the Jizantang Paleontological Museum, Liaoning Province, China) for providing the research specimen. We thank Irina Ruf (Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Germany) and Guillermo W. Rougier (Department of Anatomical Sciences and Neurobiology, University of Louisville, Louisville, KY, USA) for constructive and detailed comments on an earlier version of this study. We are grateful to Irina Ruf for sharing her research in an unpublished manuscript and especially to Guillermo W. Rougier, whose comments guided us to the interpretation of the PISM as a homologue of the interhyal. This study was supported by a grant from the National Natural Scientific Foundation of China (41202014), Public Science and Technology Research Funds Projects of Land and Resources, China (201311120), and the American Museum of Natural History. Finally, we would like to express our gratitude to Łucja Fostowicz-Frelik (Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland) and Richard L. Cifelli (Oklahoma Museum of Natural History, Norman, OK, USA) for the honor of inviting us to contribute our work to this special volume that celebrates the life of Zofia Kielan-Jaworowska, an inspiration to all of us who have been unearthing the origin and evolution of mammals, as well as the meaning of life itself. MATERIAL AND METHODS The holotype specimen of Chaoyangodens lii Hou et Meng, 2014 (JZT005-2010) is a skeleton from the lacustrine beds of